Of the ink-jet recording apparatuses that perform recording on a recording medium by ejecting ink droplets directly onto the recording medium from nozzles, those of an on-demand type that eject ink only when necessary do not require a mechanism for collecting ink. Therefore, the on-demand-type ink-jet recording apparatuses can be reduced in cost and size, and have features that can support color recording.
The ink-jet recording apparatuses are employed as image-recording apparatuses or image-forming apparatuses such as printers, facsimile machines, copiers, and plotters. An ink-jet head, which is a liquid droplet ejecting head employed in the ink-jet recording apparatuses, includes: one or a plurality of nozzles for ejecting ink droplets; one or a plurality of pressure liquid chambers (also called ejection chambers, pressure chambers, liquid chambers, and ink channels) communicating with the nozzles; and means (actuator means) for generating pressure for pressurizing ink in the pressure liquid chambers. The ink in the pressure liquid chambers is pressurized by the actuator means so that ink droplets are ejected from the nozzles.
The liquid droplet ejecting heads include those ejecting a liquid resist as liquid droplets and those ejecting a DNA sample as liquid droplets. The following description, however, focuses on the ink-jet head. An actuator forming the actuator part of the liquid droplet ejecting head is also applicable to micro devices such as a micropump, an optical device such as a micro-optical modulator, a microswitch (micro-relay), the actuator of a multi-optical lens (an optical switch), a micro-flowmeter, and a pressure sensor.
A piezoelectric ink-jet head or a bubble-type ink-jet head is popularly used as such an ink-jet head, while an electrostatic ink-jet head using electrostatic force as means for generating pressure is also known.
The piezoelectric ink-jet head, which employs electromechanical transduction, generates pressure waves in the ink chambers by the electrostatic displacement of the piezoelectric elements, thereby ejecting ink from the ink nozzles. The bubble-type ink-jet head, which employs electro-thermal transduction, generates bubbles in the ink chambers by a heater that is heated to high temperature in a short time so as to eject ink by the volume expansion of the bubbles.
Further, the electrostatic ink-jet head includes multiple actuators provided in parallel, the multiple actuators each including a pressure liquid chamber communicating with an ink nozzle hole, a diaphragm forming a wall of the pressure liquid chamber, and an electrode provided opposite the diaphragm with a predetermined minute air gap formed therebetween. In each actuator, a voltage is applied to the electrode so that the diaphragm is deformed. Thereby, pressure is generated in the pressure liquid chamber, so that the ink liquid in the pressure liquid chamber is ejected from the ink nozzle hole as a liquid droplet. In other words, the electrostatic ink-jet head deforms the diaphragm of each actuator using an electrostatic attraction force, and ejects the ink in the pressure liquid chamber from the nozzle hole by mechanical force at the time of the deformation or by mechanical repulsion generated in the diaphragm when the electrostatic attraction force disappears.
Recently, the bubble-type and electrostatic ink-jet heads, which are lead-free, have attracted attention in terms of environmental protection. Particularly, the electrostatic ink-jet head, which consumes less power in addition to being lead-free so as to have less effect on the environment, is available in a wide variety.
Further, the electrostatic ink-jet head is producible by wafer processing. Therefore, it is easy to produce a high-density ink-jet head, and it is possible to produce a large number of devices of stable characteristics. Further, the electrostatic ink-jet head, which is based on a planar structure, is characterized by easiness in reducing its size (Japanese Laid-Open Patent Applications No. 2-289351, No. 5-050601, and No. 6-071882).
Next, FIG. 1 is an exploded perspective view of a conventional electrostatic liquid droplet ejecting head 100, and FIG. 2 is a longitudinal sectional view of an actuator part of the liquid droplet ejecting head 100 in an assembled state. The liquid droplet ejecting head 100, which is, for instance, an ink-jet head employed in an ink-jet recording apparatus, includes a layer structure formed by superimposing and joining a channel substrate 101 that is a first substrate, an electrode substrate 102 that is a second substrate, and a nozzle plate 103 that is a third substrate so that the electrode substrate 102 is joined to the lower side of the channel substrate 101 and the nozzle plate 103 is joined to the upper side of the channel substrate 101.
Further, the liquid droplet ejecting head 100 includes: a plurality of nozzle holes 131 formed at appropriate positions in the nozzle plate 103 as through holes; pressure liquid chambers 111 that are ink channels communicating with the nozzle holes 131; diaphragms 113 forming wall faces of the pressure liquid chambers 111; a common liquid chamber 112 communicating with the pressure liquid chambers 111 via fluid resistance parts 115 connecting the common liquid chamber 112 and the pressure liquid chambers 111; and individual electrodes 122 provided below and opposite the diaphragms 113 with spaces (vibration chambers) 121 for deflecting the diaphragms 113 by electrostatic forces being formed therebetween. The fluid resistance parts 115 may be provided as recesses on the lower surface of the nozzle plate 103. In each actuator, a voltage applied to the electrode 122 causes a potential difference-between the electrode 122 and the diaphragm 113 so that the diaphragm 113 deflects (vibrates). Ink is ejected from the nozzle hole 131 by a pressure wave generated when the diaphragm 113 returns toward the pressure liquid chamber 111 after deflecting toward the vibration chamber 121.
The pressure liquid chambers 111 each have an elongated shape, and are provided parallel to one another, separated by partition walls 111a. The nozzle holes 131 are formed as individual through holes in the nozzle plate 103 in the parts corresponding to the pressure liquid chambers 111. When the nozzle plate 103 is joined to the upper side of the channel substrate 101, the pressure liquid chambers 111 are separated by the partition walls 111a. 
The electrodes 122, which are provided on the electrode substrate 102, are formed at the bottom of the vibration chambers 121 formed so as to correspond to the pressure liquid chambers 111 formed on the channel substrate 101. The vibration chambers 121 are partitioned by partition, walls 121a. 
The common liquid chamber 112 is provided so as to extend over the end part of each pressure liquid chamber 111. Ink is supplied from an ink tank (not shown in the drawings) to the common liquid chamber 112 via an ink supply hole (a liquid droplet supply hole) (not shown in the drawings) communicating with the lower part of the common liquid chamber 112 and other parts. The ink is further supplied from the common liquid chamber 112 via the liquid resistance parts 132 to the pressure liquid chambers 111.
The market demand for energy saving is growing for office automation equipment including ink-jet printers as well as for other electronic equipment. The electrostatic liquid droplet ejecting head, which is characterized by low power consumption compared with other types of liquid droplet ejecting heads, requires a further reduction in its driving voltage in order to achieve a further reduction in power consumption. In order to meet such a demand, the vertical dimension of the vibration chambers 121 (or the distance between the electrodes 122 and the diaphragms 113) (hereinafter this distance may be referred to as an air gap) and the thickness of the diaphragms 113 need to be reduced. This configuration can indeed lower the driving voltage. According to this configuration, however, the vibration chambers 121 have a reduced vertical dimension and the diaphragms 113 have reduced rigidity. Therefore, if moisture exists in the vibration chambers 121, the diaphragms 113 adhere to and remain in contact with the electrodes 122 through liquid bridging or hydrogen bonding, thereby preventing the actuators from functioning. Accordingly, no fluid (liquid) should be allowed into the vibration chambers 121 from the outside. For this purpose, it is desirable that the vibration chambers 121 be completely isolated from the external environment. At least, liquid such as water should be prevented from entering the vibration chambers 121.
Therefore, the openings of the vibration chambers 121 may be sealed by a sealing material so as to hermetically seal the vibration chambers 121. There arises another problem, however, in the case of employing the configuration that does not allow gas outside the liquid droplet ejecting head to enter and exit from the vibration chambers 121 and a space communicating therewith (hereinafter referred to collectively as an actuator chamber). That is, the gas inside the actuator chamber and the gas outside the head cannot freely communicate with each other, so that a change in pressure or temperature in the external environment causes a difference in pressure between the actuator chamber and the external environment, thereby changing the equilibrium positions of the diaphragms 113 in accordance with the magnitude of the pressure difference. For instance, if the pressure inside the actuator chamber is lower than the pressure outside the head, the equilibrium position of each diaphragm 113 approaches the electrode side. If the pressure inside the actuator chamber is higher than the pressure outside the head, the equilibrium position of each diaphragm 113 moves away from the electrode side. As a result, the amount and the velocity of liquid droplets ejected from the liquid droplet ejecting head vary in accordance with the pressure difference between the actuator chamber and the external environment, thereby preventing the head from maintaining stable ejection characteristics. This results in the degradation of image quality. Accordingly, it is necessary to provide some kind of correction means with respect to pressure and temperature.
The following are conventional measures to eliminate the above-described disadvantage.
According to “a method and device for driving and controlling an ink-jet head” disclosed in Japanese Laid-Open Patent Application No. 11-286109 (hereinafter referred to as first prior art), an external pressure is read by a pressure sensor employed as pressure detecting means or is manually input, and the waveform of a driving voltage is changed in accordance with the read or input external pressure.
According to “an electrostatic actuator and a liquid jetting apparatus using the same” disclosed in Japanese Laid-Open Patent Application No. 2001-300421 (hereinafter referred to as second prior art), a thin plate communicating with the atmosphere called a displaceable (deformable) plate is provided to a substrate called a cavity plate in which substrate diaphragms are formed, and a pressure compensation chamber (pressure correcting chamber) is formed across the displaceable plate from the atmosphere (or an atmospheric pressure chamber open to the atmosphere) so as to communicate with vibration chambers so that the pressure in the vibration chambers is compensated by the deflection of the displaceable plate. The displaceable plate forming a wall face of the pressure compensation chamber has a rigidity lower than that of each diaphragm so as to be displaced in accordance with the external atmospheric pressure.
According to the first prior art, however, storage means for storing the relationship between pressure and driving voltage waveform for compensating for a variation in the ink-jet characteristics and control means are further required in addition to the pressure detecting means, thus inevitably increasing the cost of products.
According to the second prior art, even if the actuator chamber is hermetically isolated from the atmosphere, the deformation of the diaphragms can be controlled not by a change in the equilibrium position of each diaphragm but by a great change in the equilibrium position of the displaceable plate when a difference is generated between the pressures inside and outside the actuator chamber.
The ink-jet head of the second prior art, although incurring a slight increase in its size, reduces a variation in the equilibrium position of each diaphragm only by its configuration. Therefore, unlike the method of providing a pressure sensor, for instance, which method cannot be expected in principle to produce a desired effect, the ink-jet head of the second-prior art is expected to produce a sufficient effect.
This configuration, however, also includes another problem due to the fact that the rigidity of the displaceable plate is sufficiently lower than that of each diaphragm. That is, if the distance between the displaceable plate and its opposing surface is small, the displaceable plate easily comes into contact with the opposing surface due to the generation of the difference between the pressures inside and outside the head. At this point, once the displaceable plate comes into contact with the opposing surface, the displaceable plate, whose rigidity is extremely low, sticks thereto due to the van der Waals force exerted on the displaceable plate and its opposing surface so as to lose its function. Further, if absorption water or a residual electric charge exists between the displaceable plate and its opposing surface, the displaceable plate sticks to its opposing surface more easily.
On the other hand, if the distance between the displaceable plate and its opposing surface is large, the displaceable plate is prevented from coming in contact with its opposing surface. Therefore, the displaceable plate is prevented from sticking to its opposing surface, but the volume of the pressure compensation chamber is increased. That is, the volume of the actuator chamber is significantly increased, so that the difference between the pressures inside and outside the actuator chamber exerts a more significant effect. As a result, a larger area is required as a space for the pressure compensation chamber, thus increasing the size and cost of the head and leading to an increase in the size of a printer using the head. This is not preferable in terms of space saving.
Thus, in the conventional electrostatic actuator and the ink-jet head using the same of the second prior art, the sticking of the displaceable plate cannot be prevented by reducing the distance between the displaceable plate and its opposing surface without incurring an unnecessary increase in the size of the head. That is, the stable ejection or operation characteristics cannot be obtained.
As described above, the vertical dimension of a vibration chamber formed between a diaphragm forming a wall of a pressure liquid chamber and the corresponding electrode of an electrostatic ink-jet head is not more than a few microns. If the vibration chamber is left open to the atmospheric environment, dust may enter the vibration chamber so as to prevent the diaphragm from deforming. Further, if moisture adheres to the surface of the diaphragm, the diaphragm may adhere to the electrode through liquid bridging, thus causing ejection failure. Furthermore, when the diaphragm is driven continuously, the vibration chamber may gradually lose the gas inside so as to enter a depressurized state. Then, even if no voltage is applied to the electrode, the diaphragm may deflect toward the electrode side never to return to its equilibrium position, thus resulting in an insufficient amount of or insufficient pressure for ink ejection. Normally, therefore, a part open to the atmosphere which part communicates with the vibration chamber is sealed by resin so that the vibration chamber is hermetically sealed.
However, in the case of hermetically sealing the vibration chamber interposed between the diaphragm and the electrode in an environment, for instance, where the atmospheric pressure is extremely different from its normal state as in highlands where the atmospheric pressure is lower than its normal value, the diaphragm is kept deflected toward the pressure liquid chamber side by the pressure difference between the pressure inside the vibration chamber and the low external pressure, thus resulting in ejection failure. If the external pressure is higher than the pressure inside the vibration chamber, the diaphragm is kept deflected in the opposite direction.
Therefore, according to the technologies disclosed in Japanese Laid-Open Patent Applications No. 11-286109 and No. 2000-272120, the difference between the pressure inside a vibration chamber and the external atmospheric pressure is measured by the pressure detecting means so as to correct the waveform of the driving voltage, and a vibration plate of a large area for pressure control is additionally provided so as to change the volume of a hermetically sealed part. Thereby, the difference between the pressure inside the vibration chamber and the external atmospheric pressure is controlled. However, the addition of the pressure detecting means or the large-area pressure control means adds to the cost of the head and makes chip downsizing and integration difficult.